Various kinds of optical instruments are commonly employed in various surveying and construction applications and are used for measuring distances and/or angles of objects or for indicating a direction or plane by a laser. Generally an optical instrument is first set up in an absolutely level position in an initial leveling procedure. After the instrument is in the level position, the actual surveying or indicating procedure can start, including calculating any angles of an emitted laser beam of the optical instrument under the condition that the instrument is in the level position.
For example, in construction applications, a laser beam emitted by the instrument can be used as an elevational reference over a work area such as for example a construction site. If a laser beam emitter is arranged to rotate the laser beam in a plane, the rotating laser beam can be used to establish a reference work plane as an elevational reference over a two-dimensional work area. Laser beam detectors placed remote from the instrument intercept the laser beam for guidance. For example the laser beam detectors are carried by earth moving equipment to determine proper elevation at various points throughout the work area. If the laser beam or the reference work plane is tilted by a known angle or angles for example to a horizontal direction, various kinds of slopes can be indicated by way of the laser.
However, especially under field conditions, e.g. outdoors at a construction site, the positioning of an optical instrument and thus also its operation may not be simple. Uneven and muddy ground may lead to a tilting of the instrument which is usually corrected manually. Moreover, an initial leveling procedure may be time consuming.
More precisely, optical instruments with angle indicators are known, which provide an operator with information about the inclination of the optical instrument. For years, bubble vials have been used as level indicators so that the operator may adjust a stand of an optical instrument, e.g. a tripod, to bring the instrument into an leveled operation state. This often requires changing the position and the length of the legs of the tripod until the bubble indicator indicated that the instrument is leveled.
Bubble vials may be used in combination with a light emitting element and a photo sensor so that an electronic signal indicating the leveling state may be provided. Further, a bubble level or vial usually only allows for indicating a very small angle of about ±1 or 2 degrees from a horizontal plane perpendicular with respect to gravity.
A bubble level is thus capable of indicating the level of a surface between horizontal and vertical to usually the nearest degree, e.g. the level of a surface of a base portion of an optical instrument with respect to gravity. Such level indicators are therefore suitable if an initial leveling procedure is carried out. However, bubble indicators are not suitable for assessing larger inclinations of the optical instrument, e.g. if the instrument is not in an absolutely level position or if over time the instrument tilts and leaves an absolutely leveled position such as on soft ground.